This application claims priority from Japanese Application 2001-005,944, filed Jan. 15, 2001, and Japanese Application 2001-387,772, filed Dec. 20, 2001, the entireties of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an epitaxial base substrate and an epitaxial substrate, particularly usable as an epitaxial substrate and the base of the epitaxial substrate for a semiconductor light-emitting element made of a plurality of III nitride films.
2. Related Art Statement
III nitride films are used as semiconductor films for light-emitting diodes, and recently get attention as semiconductor films for high speed IC chips.
Such a III nitride film is formed as follows. First, a buffer film is formed from a III nitride material on a substrate made of a single crystal material, and an underlayer is formed from a III nitride material on the buffer film. Then, one or more III nitride films having their respective functions are formed on the underlayer, to fabricate a semiconductor element such as a light-emitting diode. Generally, the substrate, and the buffer film and the underlayer are combined, and called an xe2x80x9cepitaxial substratexe2x80x9d.
The III nitride films encompassing the buffer film and the underlayer are usually made by MOCVD methods. In the case of making such an epitaxial substrate by MOCVD methods, however, the epitaxial substrate may be warped largely. Also, this trend becomes conspicuous as the buffer film is made of a III nitride material including much Al element, particular AlN.
Practically, if the buffer film is made of AlN, the warpage of the epitaxial substrate is increased up to 70 xcexcm, and thus, in the manufacturing process of semiconductor elements encompassing various semiconductor films formation processes, the photolithography process can not be performed precisely, so that the manufacturing yield ratio of semiconductor element can be deteriorated.
It is an object of the present invention to provide an epitaxial base substrate by which a not much warped epitaxial substrate can be provided, and an epitaxial substrate including the epitaxial base substrate.
In order to achieve the above object, this invention relates to an epitaxial base substrate comprising:
a base made of a single crystal material, and
a III nitride film including at least Al element and having a screw-type dislocation density of 1xc3x97108/cm2 or below which is formed on said base.
Also, this invention relaxes to an epitaxial substrate comprising:
a base made of a single crystal material,
a III nitride buffer film including at least Al element and having a screw-type dislocation density of 1xc3x97108/cm2 or below which is formed on said base, and
a III nitride underfilm which is formed on said III nitride buffer film.
The inventors had intensely studied to find out the cause of the increase in warpage of the epitaxial substrate as being fabricated by a MOCVD method. As a result, they found out the following fact of the matter.
In the past, a buffer film of an epitaxial substrate is formed at a lower temperature within 500-700xc2x0 C. so as to mitigate the difference in lattice constant between a substrate and an underfilm, and thus, enhance the buffer effect. Therefore, the crystal quality may be degraded to some degree, and large dislocation of a density of 109/cm2 or more may be created therein.
On the contrary, the inventors found out that, if the buffer film is made of a nitride including Al element and a given III nitride film is formed on the buffer film, surprisingly, the warpage of the thus obtained multilayered structure is reduced remarkably even though the dislocation density of the buffer film is reduced and thus, the crystal quality of the buffer film is developed.
Moreover, the buffer film made of the Al-including nitride can exhibit a sufficient buffer effect even though the crystal quality is developed to some degree.
The present invention is achieved by the prominent attention of the inventors which breaks down conventional ideas.
According to the epitaxial base substrate and the epitaxial substrate of the present invention, the warpage of a given multilayered structure made of a substrate and a III nitride film can be reduced, as compared with a conventional one. Therefore, when various III nitride films having their respective functions are formed on the III nitride film as an underlayer, the warpage of the resulting multilayered structure can be reduced up to 50 xcexcm or below.
Therefore, in the lithograph step of the manufacturing process of a semiconductor element, a multilayered substrate of e.g., a base substrate, a buffer film and an underlayer can be set to a desired position precisely, so the lithograph step can be performed precisely. As a result, the manufacturing yielding ratio of the semiconductor element can be developed remarkably.
The term xe2x80x9cwarpagexe2x80x9d means a xe2x80x9cwarpage amplitudexe2x80x9d of a multilayered structure such as an epitaxial base substrate and an epitaxial substrate if the multilayered structure is warped convexly or concavely, as shown in FIG. 1, and a xe2x80x9cmaximum warpage amplitudexe2x80x9d thereof if the multilayered structure is warped in a wave shape.